Acrolein is the simplest of the unsaturated aldehydes. It is also known as 2-propenal, acrylaldehyde or acrylic aldehyde. As a result of its structure, acrolein has high reactive power by virtue of the presence of its two reactive functions, which are capable of reacting individually or together. It is for this reason that acrolein finds many applications, especially as a synthetic intermediate. It is in particular a key intermediate for the synthesis of methionine, a synthetic protein used as an animal feed supplement, which has established itself as a substitute for fishmeal. Acrolein is a non-isolated synthetic intermediate of acrylic acid in the industrial production of acrylic acid by catalytic oxidation of propylene in the gas phase. The importance of the chemistry of acrylic acid and its derivatives is known. Acrolein also leads, via reaction with methyl vinyl ether followed by hydrolysis, to glutaraldehyde, which has many uses in leather tanning, as a biocidal agent in oil well drilling and during the processing of cutting oils, and as a chemical disinfectant and sterilizing agent for hospital equipment.
Acrolein is usually used as a synthetic intermediate of derivatives that are synthesized on the site of production to minimize the transportation of acrolein from the manufacturer to the client. The essential reason is linked to the toxicity of acrolein, which leads industrials to avoid the storage and transportation of this chemical product.
The most commonly used process for producing acrolein is based on the gas-phase catalytic oxidation reaction of propylene with atmospheric oxygen. The acrolein thus obtained may then be incorporated directly into an acrylic acid manufacturing process. When acrolein is used as starting material for the synthesis of methionine or for fine chemistry reactions, a purification section allows the removal of the reaction by-products, mainly carbon oxides, acrylic acid, acetic acid and acetaldehyde.
The production of acrolein is thus highly dependent on the propylene starting material obtained by steam cracking or catalytic cracking of petroleum fractions. This starting material, of fossil origin, furthermore contributes towards increasing the greenhouse effect. It thus appears necessary to have available an acrolein synthesis process that is not dependent on propylene as resource and that uses another starting material, which is preferably renewable. This process would be particularly advantageous for the synthesis of methionine, which might then be said to be “obtained from biomass”. Specifically, during its use in animal feed, methionine is rapidly metabolized and the carbon dioxide expelled into the atmosphere contributes towards increasing the greenhouse effect. If acrolein is obtained from a renewable starting material, for example obtained from plant oil, the CO2 emissions no longer enter into the process balance, since they compensate for the carbon dioxide used by the biomass for its growth; there is therefore no increase in the greenhouse effect. Such a process thus satisfies the criteria associated with the new concept of “green chemistry” within a more global context of durable development.
It has been known for a long time that glycerol can lead to the production of acrolein. Glycerol (also known as glycerine) is derived from the methanolysis of plant oils at the same time as the methyl esters, which are themselves used especially as fuels or combustibles in diesel and domestic fuel oil. It is a natural product that has an “environmentally friendly” image, is available in large amount and may be stored and transported without difficulty. Many studies have been devoted to the financial upgrading of glycerol according to its degree of purity, and the dehydration of glycerol to acrolein is one of the routes envisaged.
The reaction involved for obtaining acrolein from glycerol is:CH2OH—CHOH—CH2OH?CH2═CH—CHO+2H2O
As a general rule, the hydration reaction is favoured at low temperatures, and the dehydration reaction is favoured at high temperatures. To obtain acrolein, it is thus necessary to use a sufficient temperature, and/or partial vacuum to shift the reaction. The reaction may be performed in the liquid phase or in the gas phase. This type of reaction is known to be catalysed by acids.
According to patent FR 695 931, acrolein is obtained by passing glycerol vapours at a sufficiently high temperature over salts of acids containing at least three acid functions, for instance phosphoric acid salts. The yields indicated are greater than 75% after fractional distillation.
In U.S. Pat. No. 2,558,520, the dehydration reaction is performed in the gas/liquid phase in the presence of diatomaceous earths impregnated with phosphoric acid salts, suspended in an aromatic solvent. A degree of conversion of the glycerol into acrolein of 72.3% is obtained under these conditions.
The process described in patent application WO 99/05085 is based on a complex homogeneous catalysis, under a CO/H2 atmosphere at a pressure of 20/40 bar and in the presence of a solvent such as an aqueous solution of sulfolane.
Chinese patent application CN 1 394 839 relates to a process for preparing 3-hydroxypropanaldehyde from glycerol. The acrolein produced as reaction intermediate is obtained by passing vaporized pure glycerol over a catalyst of potassium sulfate or magnesium sulfate type. The reaction yields are not given.
U.S. Pat. No. 5,387,720 describes a process for producing acrolein by dehydration of glycerol, in the liquid phase or in the gas phase over acidic solid catalysts defined by their Hammett acidity. The catalysts must have a Hammett acidity of less than +2 and preferably less than −3. These catalysts correspond, for example, to natural or synthetic siliceous materials, for instance mordenite, montmorillonite, acidic zeolites; supports, such as oxides or siliceous materials, for example alumina (Al2O3), titanium oxide (TiO2), coated with mono-, di- or triacidic inorganic acids; oxides or mixed oxides such as gamma-alumina, the mixed oxide ZnO—Al2O3, or alternatively heteropolyacids. According to the said patent, an aqueous solution comprising from 10% to 40% of glycerol is used, and the process is performed at temperatures of between 180° C. and 340° C. in the liquid phase, and between 250° C. and 340° C. in the gas phase. According to the authors of the said patent, the gas-phase reaction is preferable since it enables a degree of conversion of the glycerol of close to 100% to be obtained, which leads to an aqueous acrolein solution containing side products. A proportion of about 10% of the glycerol is converted into hydroxypropanone, which is present as the major by-product in the acrolein solution. The acrolein is recovered and purified by fractional condensation or distillation. For a liquid-phase reaction, a conversion limited to 15-25% is desired, to avoid excessive loss of selectivity. U.S. Pat. No. 5,426,249 describes the same gas-phase process for the dehydration of glycerol to acrolein, but followed by a hydration of the acrolein and a hydrogenation to lead to 1,2- and 1,3-propanediol.
The dehydration reaction of glycerol to acrolein is thus generally accompanied by side reactions leading to the formation of by-products such as hydroxypropanone, propanaldehyde, acetaldehyde, acetone, adducts of acrolein with glycerol, glycerol polycondensation products, cyclic glycerol ethers, etc., but also phenol and polyaromatic compounds, which are the cause of the formation of coke on the catalyst. This results, firstly, in a reduction in the yield of and the selectivity towards acrolein, and secondly in deactivation of the catalyst. The presence of by-products in the acrolein, such as hydroxypropanone or propanaldehyde, some of which are moreover difficult to isolate, necessitates separation and purification steps, which lead to high recovery costs for the purified acrolein. Moreover, it is necessary to regenerate the catalyst very regularly in order to regain satisfactory catalytic activity.